Preparation of photographic emulsion containing narrow size distribution of octahedral silver halide grains sensitized with gold compound

ABSTRACT

A PROCESS FOR PREPARING A SILVER HALIDE PHOTOGRAPHIC EMULSION HAVING A VERY NARROW DISTRIBUTION OF GRAIN SIZES AND A REDUCED FOG FORMATION ON BEING SENSITIZED WITH A GOLD-CONTAINING CHEMICAL SENSITIZER COMPRISING ADDING A WATER-SOLUBLE SILVER SALT SOLUTION AND A WATER-SOLUBLE ALKALI METAL HALIDE SOLUTION SIMULTANEOUSLY AND GRADUALLY TO AN AQUEOUS SOLUTION OF GELATIN WHILE MAINTAINING THE CONCENTRATION OF SILVER ION AT A CONSTANT VALUE WITHIN A RANGE OF 10**-6 TO 10**-10 MOL/LITER, REDUCING THE SILVER ION CONCENTRATION TO LESS THAN 1/2 THE PREVIOUS CONCENTRATION USED AFTER ADDING 1/20 TO 1/2 OF THE TOTAL SILVER SALT, CONTINUING THE ADDITION AT THIS CONCENTRATION, RINSING THE EMULSION FREE OF WATER-SOLUBLE SALTS, AND CHEMICALLY SENSITIZING THE EMULSION WITH A CHEMICAL SENSITIZER CONTAINING AT LEAST A GOLD COMPOUND IS DISCLOSED. PHOTOGRAPHIC ELEMENTS CONTAINING THE SILVER HALIDE EMULEION ARE ALSO DISCLOSED.

United States Patent O R v 16 Claims U.s. c1. 96-108 ABSTRACT OF THE DISCLOSURE A process for preparing a silver halide photographic emulsion having a very narrow distribution of grain s1zes and a reduced fog formation on being sensitized with a gold-containing chemical sensitizer comprising adding a water-soluble silver salt solution and a water-soluble alkali metal halide solution simultaneously and gradually to an aqueous solution of gelatin while maintaining the concentration of silver ion at a constant value within a range of to 10- mol/liter, reducing the silver ion concentration to less than V: the previous concentration used after adding to A of the total silver salt, continuing the addition at this concentration, rinsing the emulsion free of water-soluble salts, and chemically sensitizing the emulsion with a chemical sensitizer containing at least a gold compound is disclosed. Photographic elements containing the silver halide emulsion are also disclosed.

BACKGROUND OF THE INVENTION (1) Field of the invention This invention relates to a process for the preparation of a silver halide emulsion having a very narrow distribution of grain sizes and a reduced fog formation when sensitized with a chemical sensitizing agent containing at least a gold compound.

(2) Descrpition of the prior art A pure silver bromide emulsion having orderly crystal shapes and a very narrow distribution of grain sizes can be obtained using a method in which the maximum number of nuclei are formed in an early stage of the precipi- 'tation. Subsequent addition of reagents only results in an increase in the size of the nuclei and does not lead to the formation of new nuclei. Increasing the size of the nuclei without forming new nuclei can be effected by simultaneously and gradually adding a water-soluble silver salt and a water-soluble alkali metal halide and while maintaining the concentration of the silver ion in the emulsion at a constant value between 1X10 and 1X10- mol/liter (this corresponds to a pAg value of 6 to 10).

Formation of new nuclei depends upon the distances between the grans, the reaction temperature, the solubility of the silver halide, and the rate of the addition of reagents during precipitation. It is known that when precipitation proceeds at near a pAg value corresponding to the stoichiometrically equivalent amounts of the silver ion and the halogen ion, silver halide crystals with cubic faces are obtained. When precipitation proceeds in the presence of excess halogen ion (i.e. at a high pAg value), crystals with octahedral faces are obtained. When precipitation proceeds in the presence of the lower excess halogen ion i.e., near a pAg value corresponding to the stoichiometrically equivalent amounts of the silver ion and the halogen ion, silver halide crystals with cubic faces are obtained whereas crystals with almost octahedral faces are obtained in the presence of the higher excess halogen ions. Though precipitation proceeds in the presence of the higher excess halogen ions, a small quantity of crystals with cubic faces are also obtained since the chemical reaction is not discontinuous.

Our research work has led to the discovery that when precipitation is elfected within a high pAg range from the beginning of the preparation of crystals of a mixed silver halide, such as silver iodobromide and silver chloroiodobromide, crystals having a narrow distribution of grain sizes cannot be obtained. It is believed that this is due to crystallographical stacking defects occurring to form flat plate-like grains ascribable to twin crystals, and new nuclei are formed due to a reduction in solubility of the silver halide. Therefore, in order to prepare silver iodobromide or silver chloroiodobromide gains having a narrow distribution of grain sizes, reagents must be added at a pAg value within a range substantially corresponding to the stoichiometrically equivalent concentrations of the silver ion and the halogen ion in the emulsion, as described hereinbefore.

However, it has been found that when the resulting silver iodobromide or silver chloroiodobromide photographic emulsion containing cubic grains and having a very narrow distribution of grain sizes is rinsed free of the water-soluble salts and then sensitized with a chemical sensitizer containing at least a gold compound so as to increase its photosensitivity to a maximum, excessive fog is formed even with that part of the emulsion which has undergone only insuflicient chemical ripening.

Accordingly, an object of the present invention is to provide a process for the preparation of a silver halide emulsion having a very narrow distribution of grain sizes and a reduced fog formation when sensitized with a chemical sensitizer containing at least a gold compound.

Another object of this invention is to provide a gelatinsilver halide emulsion comprising silver halide grains having a narrow distribution of photosensitivity among the grams.

Still another object of the invention is to provide an emulsion suitable for use in a photographic material use ful in reversal color and black-and-white photographs.

SUMMARY OF THE INVENTION As a result, it has been discovered that a mixed silver halide photographic emulsion having a very narrow distribution of grain sizes can be obtained by reducing the concentration of the silver ion in the emulsion to less than one-half of the concentration used at the early stage in the precipitation after the formation of stable nuclei subsequent to the start of precipitation, continuing to add the alkali metal solution and the silver salt solution to form a mixed silver halide photographic emulsion, rinsing the emulsion free of any water-soluble salts, and then sensitizing the emulsion with a chemical sensitizer containing at least a gold compound. When subjected to the same exposure and development, such a silver halide emulsion can attain maximum sensitivity essentially without any fog formation, in comparison with the emulsion, described previously, which was prepared at a constant silver ion concentration during grain growth.

DETAILED DESCRIPTION OF THE INVENTION Although the mechanism of this emulsion preparation is not completely understood, it is believed that the following two reasons are important. Firstly, this emulsion, which does not suffer from fog formation, is obtained by keeping the silver halide crystals as far as possible from a reducing atmosphere during grain forming reaction (namely, avoiding the occurrence of a reduction sensitization during the formation of the grains). Secondly, while silver iodobromide or silver chloroiodobromide crystals (having a narrow distribution of sizes) obtained at a low pAg are generally a mixture of cubic grains and octahedral grains, the grains prepared by the process of the present invention consist substantially of octahedral grains.

The preparation of a silver halide photographic emulsion in accordance with the present invention is described hereinafter in greater detail.

A solution of silver nitrate and a solution of an alkali metal halide are added, simultaneously and gradually, to an aqueous solution comprising gelatin or a gelatin derivative, such as succinic gelatin, maleic gelatin or phthalic gelatin, with vigorous stirring. Ammonia at a concentration of from 1.5 X to 1.5 N can be added in advance to the aqueous solution of gelatin to obtain the desired silver halide grain sizes. During the reaction, the component solutions are added while maintaining the concentration of silver ion in the gelatin or gelatin derivative solution within the range of from 1 l0- to 1x10- mol/liter. After the addition of from to /2 of the total silver salt solution, the concentration of silver ion in the reaction solution is reduced to less than /2 of that used at an early stage in the precipitation, and the addition is continued. Preferably, the emulsion comprises a mixed silver halide containing from 0.2 to 10 mol percent, particularly from 0.5 to 7 mol percent of iodide. Best results can be obtained by adding a silver nitrate solution and an alkali metal halide solution simultaneously to the gelatin or the gelatin derivative solution over a period of from 6 to 60 minutes. After removing any watersoluble salts by rinsing, the resulting emulsion is chemically sensitized at high temperatures by adding a chemical sensitizer containing at least a gold compound.

Solutions of an alkali metal halide and silver nitrate are added to an aqueous solution comprising the gelatin or the gelatin derivative at a concentration of from 0.5 to 5.0%. The resulting silver halide dispersion is cold set, and then washed with water by diffusion or by using the coacervation technique, such as disclosed in US. Pat. 2,489,341.

If desired, an aqueous solution of an alkali metal bromide and an aqueous solution of an alkali metal iodide can be added simultaneously to the gelatin solution. This procedure minimizes excessive halogen ion in the reaction solution.

One of the embodiments of the present invention is the chemical sensitization of the emulsion thus obtained with a chemical sensitizer containing at least a gold compound, such as potassium chloroaurate (KAuCl auric trichloride (AuCl potassium aurithiocyanate (KA11(CNS).,), and potassium aurothiocyanate (KAu(CNS) At this point, a sulfur sensitizing agent, such as sodium thiosulfate pentahydrate, allyl thiocarbamide, thiourea, allyl isothiocyanate, ammonium and/or a metal thiocyanate, and/or a reduction sensitization agent, such as stannous chloride, can be used in conjunction with the gold compound (see, for instance, C. E. K. Mees and T. H. James, The Theory of the Photographic Process, 3rd edition, 114-116, Macmillan Co. (1966). The gold compound, the sulfur sensitizing agent and the reduction sensitization agent can be used at a level of from 0.1 to 10 mm. liter, from 1 to 20 mm. liter and from 1 to 10 mm. liter of aqueous solution (0.1 percent by weight), respectively, per mole of the silver halide. The emulsion can also be sensitized with a gold compound and in conjunction with it, polyethylene glycol, as described in US. Pats. 2,423,- 459 and 2,441,389. The chemical sensitization of the emulsion is effected by ripening at high temperatures for a period sufiicient to elevate the photosensitivity.

The emulsion so prepared is coated on a base of a material, such as cellulose triacetate, cellulose acetate propionate, or polyethylene terephthalate.

The emulsion obtained in accordance with the present invention having a reduced fog formation and a narrow distribution of grain sizes is suitable, particularly, for use in reversal color photographic materials or reversal blackand-white photographic materials.

The invention will be illustrated in detail by reference to the following examples "including an experimental example (for comparison). The sensitivity is expressed by the amount of exposure necessary to obtain a fog density of +0.1. The development was performed using a blackand-Whitc developer having the following composition at 27 C. for 4 minutes.

DEVELOPER COMPOSITION N-methyl-p-aminophenol sulfate g 1 Anhydrous sodium sulfite g Hydroquinone g 10 Sodium carbonate monohydrate g 40 Potassium bromide ..g 5 Potassium thiocyanate g 1.5 Potassium iodide (0.1%) ml 5 Sodium hydroxide 1 2 Water to make 1000 ml.

EXPERIMENTAL EXAMPLE 1 Solution I Twenty grams of photographic'gelatin and 3.0 g. of potassium iodide were dissolved in 1200 ml. of distilled water at 50 C., and 80 ml. of 25% aqueous ammonia was added to the mixture.

prepared.

Solution II Potassium bromide (70.1 g.) was dissolved in distilled water at 45 C. and the amount of the solution was adjusted to 400 ml.

' Solution III One hundred grams of silver nitrate was dissolved in distilled water at 45 C., and the amount of the solution was adjusted to 400 ml. 7 1."

Solutions II and III were added gradually and simultaneously to Solution I, with vigorous stirring, over a period of about 40 minutes. During-the reaction, the concentration of the silver ion in the reactionsolution was maintained at 10* mol/ liter. After the addition, the mixture was cooled to 30 C., and acidified with 6 N sulfuric acid. The emulsion was rinsed free of the water-soluble salts using conventional coacervati-on. techniques, and

, heated to a temperatureof 40 C. This was followed by complete dissolution with the addition of vg. ofphotographic gelatin. The pH and .pAg were adjusted to 6.50 8.50, respectively, with 5%v sodium hydroxide and 2% potassium bromide. To the silver halide emulsion thus obtained were added 6 ml of sodium thiosulfate pentahydrate (0.1%) and 2.5 ml. of potassium chloroaurate (0.1% by weight) per mol of the silver halide, and chem ical ripening was effected at 50 C. for 30 to minutes until the emulsion attained maximum sensitivity. After adding a coating assistant such as saponin, the emulsion was coated on an acetate film, and dried. The mean projection area; of the grainswas 0.55 3.

EXAMPLE 1 ,This example illustrates the preparation of an emulsion in accordancewith the present invention which has a very narrow distribution ofgrain sizes, Stab lity and re? duced fog formation when chemically sensitized with a chemical sensitizer containing a gold compound.

The following Solutions I, II and III were prepared.

Solution I Twenty grams of photographic gelatin and 3.0 g. of potassium iodide were dissolved in 1200 ml. of distilled water at 50 C., 80 ml. of 25% aqueous ammonia was added to the mixture.

Solution II Potassium bromide (70.1 g.) was dissolved in distilled water at 45 C., and the amount of the solution was adjusted to 400 ml.

I I Solution III One hundred grams of silver nitrate was dissolved in distilled water at 45 C., and the amount of the solution was adjusted to 400 ml.

Solutions II and III were added gradually and simultaneously to Solution I with vigorous stirring. For 10 minutes after the start of the addition, of each of the Solutions II and III were added, and during this time, the concentration of the silver ion in the reaction solution was maintained at 10" mol/liter. At the end of the 10- minute period, the silver ion concentration of the reaction solution was reduced to 10- mol/liter, and the remainder of Solutions II and III was added over a period of about 30 minutes. The resulting mixture was cooled to 30 C., and acidified with 6 N sulfuric acid. The emulsion was rinsed free of water-soluble salts using a conventional coacervation technique, and heated to 40 C. This was followed by complete dissolution with addition of 80 g of photographic gelatin. The pH and pAg were adjusted, respectively, to 6.50 with sodium hydroxide and 8.50 with 2% potassium bromide. The mean projection area of the grains so obtained was about 0.55%.

To the silver halide emulsion thus obtained were added 9 ml. of sodium thiosulfate pentahydrate (0.1%) and 2.5 ml. of potassium chloroaurate (0.1% by weight) per mol of the silver halide. Chemical ripening was effected at 50 C. for about 30 to 90 minutes until the emulsion attained maximum sensitivity. After adding a coating assistant such as saponin, the emulsion was coated on an acetate film, and dried.

EXAMPLE 2 Each of the silver halide photographic emulsions prepared in the manner set forth in Experimental Example 1 and Example 1 and coated on a film, was exposed to a light source of 5400 K. through a'continu'ous wedge using a sensitometer (Toshiba N'SG II type). The film strip was developed for 4 minutes at 27 C. with the above-described black-and-white developer containing Metol and hydroquinone. The relation of the chemical ripening time of the emulsion described in Experimental Example I and Example I to fog and relative sensitivity after development is shown in Table ,1.

TABLE 1.COATE'D FILMS Chemical When the sample of Experimental Example 1 was chemically ripened until the relative sensitivity became 100, a fog level of 0.42 was formed. In contrast to this, with the sample of Example 1, a relative sensitivity of 100 was attained at a markedly lower fog level (0.08) than the sample of Experimental Example 1.

EXAMPLE 3 Solutions I, II and HI were prepared as follows:

Solution I Twenty grams of photographic gelatin and 3.0 g. of potassium iodide were dissolved in 1200 ml. of distilled water at 50 C. 10 ml. of 25% aqueous ammonia was added to the mixture. 1

Solution II Potassium bromide (70.1 g.) was dissolved in distilled water at 45 C., and the amount of the solution was adjusted to 400 ml.

Solution III One hundred grams of silver nitrate were dissolved in distilled water at 45 C., and the amount of the solution was adjusted to 400 m1.

Solutions II and III were added, gradually and simultaneously, to Solution I, with vigorous stirring. First, 100 ml. of each of Solutions II and III were added over a period of 10 minutes while maintaining the concentration of silver ion in the reaction solution at'10 mol/ liter. At the end of the 10-minute period, the silver ion concentration was reduced to 10 mol/liter and the remainder of the Solutions II and III were added over a period of 30 minutes at this silver ion concentration. The mean projection area of the silver halide grains thus obtained was O.2,u. The emulsion was cooled to 30 C. and acidified with 6 N sulfuric acid. The emulsion was then rinsed free of water-soluble salts using a conventional coacervation technique, and g. of photographic gelatin was added, followed by heating to 40 C. to dissolve and disperse the emulsion completely. The pH and pAg were adjusted respectively to 6.50 with 5% sodium hydroxide and 8.50 with 2% potassium bromide.

The resulting emulsion was chemically ripened for 60 minutes at 50 C. with addition of 10.5 ml. of sodium thiosulfate pentahydrate (0.1%) and 2.5 ml. of potassium chloroaurate (0.1% by weight) per mol of the silver halide. The emulsion was then coated, dried, exposed and developed in the same manner as in Example 1. Maximum sensitivity was attained at a fog level of 0.07.

EXAMPLE 4 The same procedure as set forth in Example 1 was repeated except that potassium iodide was added to Solution II instead of to Solution I. When the emulsion obtained was chemically sensitized for 60 minutes at 50 C. with a chemical sensitizer containing the gold compound in the same manner as in Example 1, maximum sensitivity was attained at a low fog level (less than 0.10). The resulting silver halide photographic emulsion had a narrow distribution of grain sizes.

EXAMPLE 5 The same procedure as used in Example 1 was repeated except that Solution I containing 1.0 g. of potassium iodide and Solution II contained 2.0 g. of potassium iodide. When the resulting emulsion was chemically sensitized for 60 minutes at 50 C. with a chemical sensitizer containing the gold compound in the same manner as in Example 1, maximum sensitivity was attained at a low fog level (less than 0.10). The resulting silver halide photographic emulsion had a narrow distribution of grain sizes.

EXAMPLE 6 The same procedure as used in Example 1 was repeated except that 10 g. of potassium chloride additionally was added to Solution I. When the resulting emulsion was chemically sensitized for 60 minutes at 50 C. with a chemical sensitizer containing the gold compound in the same manner as in Example I, maximum sensitivity was attained at a low fog level (less than 0:10). The resulting silver halide photographic emulsion had a narrow distribution of grain sizes.

7 EXAMPLE 7 The same procedure as used in Example 1 was repeated except that g. of potassium chloride was added to Solution I and potassium iodide was added to Solution II, instead of Solution I. When-the resulting emulsion was chemically sensitized for 60 minutes at 60 C. with a chemical sensitizer containing the gold compound in the same manner as in Example I, maximum sensitivity was attained at a low fog level (less than 0.10). The resulting silver halide photographic emulsion had a narrow distribution of grain sizes.

What is claimed is:

1. A process for the preparation of a mixed silver halide photographic emulsion comprising substantially octahedral grains which comprises: 7

(1) simultaneously and graually adding an aqueous solution of a water-soluble silver salt in an aqueous solution of at least one water soluble alkali metal halide to an aqueous solution comprising a member selected from the group consisting of gelatin and a gelatin derivative while maintaining the concentration of the silver ion at a constant value within a range of from 10* to 10- mol/ liter,

(2) reducing said silver ion concentration to less than /2 the concentration of the silver ion used in Step (1) after the addition of from to Eof the total silver salt solution,

(3) continuing the addition of the silver salt solution and the alkali metal halide solution at the concentration of the silver ion used in Step (2) to form a mixed silver halide emulsion,

(4) removing any water-soluble salts from the emulsion, and

(5) chemically sensitizing the emulsion with a chemical sensitizer containing at least a gold compound,

wherein the pH during the formation of silver halide in Steps (1) to (3) is in the range of 6 to 11:8.

2. The process according to claim 1, wherein said water-soluble silver salt solution and said water-soluble alkali halide solution are added over a period of from 6 to 60 minutes.

3. The process according to claim 1, wherein the mixed silver halide formed is selected from the group consisting of silver iodobromide and silver chloroiodobromide.

4. The process according to claim 1, wherein the aqueous solution comprising a member selected from the group consisting of gelatin and a'gelatin derivative contains additionally at least one water soluble alkali metal halide.

5. The process according to claim 4, whereinthe mixed silver halide emulsion formed is a silver iodobromide v emulsion, wherein the water-soluble alkalimetal halide is,

an alkali metal bromide and wherein the aqueous solution of the member selected from the group consisting of gelatin and a gelatinderivative contains additionally at least an alkali metal iodide.

6. The process according to claim 1, wherein the mixed it 7. The process according to claim 4, wherein the mixed silver halide emulsion formed is a silver chloroiodobromide emulsion, wherein the water-soluble alkali halide is an alkali metal bromide, and wherein the aqueous solution comprising a member selected from the group con- Sisting of gelatin and a gelatin derivative contains additionally at least one alkali metal iodide.

8. The process according-to claim 4, wherein the mixed silver halide emulsion formed is a silver chloroidobrow mide emulsion, wherein the Water-soluble alkali metal halide is an alkali metal bromide, and wherein the aqueous solution comprising a member selected fromthe group consisting of gelatin and a gelatin derivative contains addi-. tionally at least one alkali metal chloride. I

9. The process according to claim 1, wherein the mixed silver halide emulsion formed is a silver chloroiodobromide emulsion, wherein the water-soluble alkali metal halide is a mixture of an alkali metal bromide, an alkali metal iodide and an alkali metal chloride.

10. The process according to claim 1, wherein the aqueous solution of a member selected from the group consisting of gelatin and a gelatin derivative additionally contains aqueous ammonia.

11. The process according to claim 1, wherein the chemical sen'sitizer is potassium chloroaurate. I

12. The process according to claim 1, wherein th chemical sensitizer. is a mixture of sodium thiosulfate pentahydrate and potassium chloroaurate.

13. The process according to claim 1, wherein the chemical sensitization is effected at 50 C., for a period of from about 30 to minutes.

14. A mixed silver halide photographic emulsion containing from 0.5 to 7 mol percent iodide prepared according to the process of claim 1.

15. The silver halide photographic emulsion according to claim 14, wherein themixed silver halide formed is se-. lected from the group consisting of silver iodobromide and silver chloroiodobromide.

16. A light-sensitive element comprising a support having thereon at least one layer containing the mixed silver halide photographic emulsion prepared by the process of claim 1.

References Cited UNITED STATES PATENTS 3,482,982. 3/1971' Miyata et al. 9".61!1 4.':7 3,565,633 4/1971 Klinger 96114.7

DAVID KLEIN, Plinia'r 'ixamma,

W. 'H. LOUIE, JRL, AssistantExaminer 

